The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium having a low light transmittance, an excellent smooth surface, a high mechanical strength and an excellent durability, a non-magnetic substrate for the magnetic recording medium, having a non-magnetic undercoat layer, and non-magnetic zirconium-containing acicular particles containing iron as a main component.
With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Various attempts have been made at both enhancing the properties of magnetic particles and reducing the thickness of a magnetic recording layer in order to improve these properties of a magnetic recording medium.
The enhancement of the properties of magnetic particles will be firstly described.
The properties which magnetic particles are required to have in order to satisfy the above-described demands on a magnetic recording medium, are a high coercive force and a large saturation magnetization.
In recent years, as magnetic particles suitable for high-power and high-density recording, there have been widely used magnetic acicular iron oxide particles coated with a cobalt compound, magnetic acicular metal particles containing iron as a main component, which are produced by heat-reducing acicular goethite particles or acicular hematite particles in a reducing gas.
A reduction in the thickness of a magnetic recording layer will now be described. Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
With respect to short wavelength signals, a reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co., Ltd. (1982), " . . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness. . . . ".
Development of a thinner film for a magnetic recording layer has caused some problems. Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the base film must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology-Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as "Materials for Synthetic Technology" (1987), published by the Publishing Department of Technology Information Center, " . . . the surface roughness of a hardened magnetic layer depends on the surface roughness of the base film (back surface roughness) so largely as to be approximately proportional, . . . , since the magnetic layer is formed on the base film, the more smooth the surface of the base film is, the more uniform and larger head output is obtained and the more the S/N ratio is improved."Secondly, there has been caused a problem in the strength of a base film with a tendency of the reduction in the thickness of the base film in response to the demand for a thinner magnetic recording layer. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, " . . . Higher recording density is a large problem assigned t the present magnetic tape. This is important in order to shorten the length of the magnetic tape so as to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a base film . . . . With the tendency of reduction in thickness of the magnetic tape, the stiffness of the magnetic tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a videotape both in the machine direction and in the transverse direction is now strongly demanded. . . . "
The end portion of a magnetic recording medium such as a magnetic tape, especially, a videotape is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large by a video deck. If the light transmittance of the whole part of a magnetic recording layer is made large by the production of a thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present videotapes.
However, addition of a large amount of non-magnetic particles such as carbon black impairs not only the enhancement of the recording density but also the development of a thinner recording layer. In order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce, as much as possible, the quantity of non-magnetic particles such as carbon black which are added to a magnetic recording layer.
It is, therefore, strongly demanded that the light transmittance of a magnetic recording layer should be small even if the carbon black or the like which is added to the magnetic recording layer is reduced to a small amount. From this point of view, improvements in the non-magnetic substrate are now in strong demand.
There is no end to a demand for a higher performance in recent magnetic recording media. Since the above-described reduction in the thickness of a magnetic recording layer and a base film lowers the durability of the surface of the magnetic recording layer and the magnetic recording medium, an improvement of the durability of the surface of the magnetic recording layer and the magnetic recording medium is in strong demand.
This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-298679, " . . . With the recent development in magnetic recording, a high picture quality and a high sound quality have been required more and more in recording. The signal recording property is, therefore, improved. Especially, finer and higher-density ferromagnetic particles have come to be used. It is further required to make the surface of a magnetic tape smooth so as to reduce noise and raise the C/N. . . . However, the coefficient of friction between the magnetic layer and an apparatus during the travel of the magnetic tape increases, so that there is a tendency of the magnetic layer of the magnetic recording medium being damaged or exfoliated even in a short time. Especially, in a videotape, since the magnetic recording medium travels at a high speed in contact with the video head, the ferromagnetic particles are apt to be dropped from the magnetic layer, thereby causing clogging on the magnetic head. Therefore, an improvement in the running stability of the magnetic layer of a magnetic recording medium is expected. . . . "
Various efforts have been made to improve the non-magnetic substrate for a magnetic recording layer with a demand for a thinner magnetic recording layer and a thinner base film. A magnetic recording medium having at least one undercoat layer (hereinunder referred to "non-magnetic undercoat layer") comprising a binder resin and non-magnetic iron-based particles such as hematite particles, iron oxide hydroxide particles or the like, which are dispersed therein, on a base film such as a base film has been proposed and put to practical use (Japanese Patent Publication (KOKOKU) No. 6-93297 (1994), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338 (1987), 63-187418 (1988), 4-167225 (1992), 4-325915 (1992), 5-73882 (1993), 5-182177 (1993), 5-347017 (1993), 6-60362 (1994), etc.)
In addition, as non-magnetic particles used in the non-magnetic undercoat layer, there are known such non-magnetic particles which surfaces are treated with a zirconium compound in order to improve the dispersibility in vehicle or the like (Japanese Patents Nos. 2,566,088, 2,571,350 and 2,582,051, and Japanese Patent Applications Laid-open (KOKAI) Nos. 6-60362(1994), 9-22524(1997), 9-27117(1997), 5-73883 (1993), 6-60360(1994), 8-50718(1996), 8-255334(1996), 9-27116(1997), 9-27117(1997) and 9-35245(1997)).
A magnetic recording medium which has small light transmittance, high strength, smooth surface and higher durability, with reduction of the thickness of not only the magnetic recording layer but also the base film is now in the strongest demand, but no such magnetic recording medium which sufficiently satisfies these conditions have ever been obtained.
The above-described magnetic recording media composed of a base film and a non-magnetic undercoat layer produced by dispersing non-magnetic particles in a binder resin and formed on a base film, have a small light transmittance, a smooth surface and a high strength, but the durability thereof is inconveniently poor.
This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993), ". . . Although the problem of surface roughness is solved by providing a magnetic layer as an upper layer after forming a thick non-magnetic undercoat layer on the surface of a base film, the problem of the abrasion of a head and the problem of durability are not solved and still remain. This is considered to be caused because a thermoset resin is usually used as a binder of the undercoat layer so that the magnetic layer is brought into contact with a head or other members without any cushioning owing to the hardened undercoat layer, and a magnetic recording medium having such an undercoat layer has a considerably poor flexibility."
As a result of the present inventors' earnest studies, it has been found that by using as non-magnetic acicular particles for a non-magnetic undercoat layer, non-magnetic acicular particles containing iron as a main component, comprising iron and zirconium of 0.05 to 30% by weight (calculated as Zr) based on the total weight of the particles, which is present within the particle, there can be obtained a magnetic recording medium having a low light transmittance, an excellent smooth surface, a high mechanical strength and an excellent durability. The present invention has been attained on the basis of this finding.